The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile system which forms a color image with the use of electrophotography, and in particular, to an electrophotographic apparatus for forming a color image with the use of a plurality of color toners.
With the electrophotography, a light beam is projected from an exposure means onto a photosensitive medium which is uniformly charged, so as to form a latent image corresponding to image data, and toner is stuck to the latent image on the photosensitive medium so as to develop the latent image. The thus obtained toner image is transferred onto and fixed on a recording medium.
It is noted here that explanation will be hereinbelow made so as to use the recording medium as a sheet. However, the recording medium should not be limited to the sheet alone but there may be used a sheet-like recording medium made of any of various materials including plastic.
In order to form a color image, a plurality of color toners such as yellow Y, magenta M, cyan C and black K are superposed one upon another so as to form the image.
There are two kinds of color image forming systems, such as a repeated developing system in which development is repeated on a single photosensitive medium with color toners so as to form a color image, and a simultaneous developing system in which developments are simultaneously carried out on a plurality of photosensitive mediums with color toners so as to form a color image.
The repeated development system is the one in which a single photosensitive medium is used for forming a color image, and as typical examples, there are a transfer drum system and an intermediate transfer medium system.
In the transfer drum system, a plurality of developing means for developing images with different color toners are arranged around a photosensitive medium, together with a transfer drum for winding a recording medium thereon, and different color images formed on the photosensitive medium are transferred one by one onto the recording medium. This operation is repeated for the different color images which are superposed one upon another on a recording medium so as to obtain a color image.
In the intermediate transfer medium system, a plurality of developing means for developing images with different color toners are arranged around a photosensitive medium, together with an intermediate transfer medium, and toner color images formed on the photosensitive medium are transferred one by one onto the intermediate transfer medium. This transfer is repeated for different color images so as to superpose these images on the intermediate transfer medium in order to form a color image which is then transferred onto a medium on which the color image is fixed.
The transfer drum system is apt to cause the quality of an obtained color image to be affected by a kind of sheets since different color toner images are directly superposed on a recording medium, that is, the intermediate transfer medium system is more excellent in view of the formation of a color image on any of various kinds of sheets including a cardboard.
However, even in the intermediate transfer medium system, since different toner color images of, for example, yellow Y, magenta M, cyan C and black K are formed one by one on the photosensitive medium, and are then transferred onto the intermediate transfer medium, being superposed one upon each other, a time which is four times as long as that required for formation of a monochromatic image is required.
The simultaneous developing system simultaneously forms color toner images respectively on a plurality of photosensitive mediums for respective different colors, and transfers the different color toner images in association with a conveyance of a sheet so as to form a color image. Thus, this developing system is also called as a tandem system.
The tandem system incorporates an image forming means including a photosensitive medium, a charging means, an exposure means, a developing means and a cleaner means for each color, and accordingly, four image forming means are required for forming a color image with color toners of yellow Y, magenta M, cyan C and black K.
In the tandem system, different toner color images are formed by four independent image forming means, simultaneously in parallel with each other, and are then transferred onto an intermediate transfer medium or a sheet. In the tandem system, since different toner color images are simultaneously superposed one upon another, a color image can be formed by a time nearly equal to that required for formation of a monochromatic image, and accordingly, this system is preferable for high speed printing of a color image.
It is noted that the tandem system is sorted into a direct transfer system and an intermediate transfer medium system in view of a diagnostic criteria whether different color images are directly superposed with one other on a recording medium or are superposed with one other a transfer medium. The intermediate transfer medium system is advantageous for different kinds of sheets, similar to the repeated development system.
These years, there have been increased demands for colorization of documents in offices and accordingly, color printers have been rapidly spread in use. Further, it has been desired to increase the printing speed, and accordingly, tandem system color printers have been spot-lighted.
However, since the tandem system color printer inevitably incorporates four image forming means, the miniaturization of the printer is difficult, that is, it has a size which is relatively larger than that of a repeated development system color printer.
In particular, in a tandem type system color printer in which image forming means are horizontally laid as conventionally usual, an increased size of the printer causes its occupied floor area to be larger in the case of installation thereof in an office.
Thus, it has been proposed a tandem system color printer in which image forming means are arranged in a vertical direction in order to decrease the size of the printer and to reduce the occupied floor area thereof.
It has been known a tandem system color printer in which a laser light source is used as an exposure means, and four image forming means are arranged in a vertical direction along a straight part of an intermediate transfer medium (as disclosed, for example, JP-A-2001-134042). In this tandem system color printer, a laser exposure means is mounted so as to be stationary, and photosensitive drums are mounted on a common member so as to be grouped in one unit in order to ensure a high degree of accuracy. Developing means are removably mounted in front of the printer.
A tandem system color printer in which LED arrays are used as the exposure means, and four image forming means are stacked in a vertical direction along a straight part of an intermediate transfer medium has been also known (as disclosed in JP-A-2001-356548). In this tandem system color printer, the LED arrays are mounted to be stationary so as to ensure a high degree of accuracy. A process cartridge in which the photosensitive drums and the developing means are integrally incorporated is removably mounted in front of the printer.
It is noted that the image forming means are arranged in a vertical direction in the printer disclosed in JP-A-2001-134042 or JP-A-2001-356548 in order to decrease the size of the printer and to reduce the occupied floor area.
In the printer disclosed in JP-A-2001-134042, the developing means can be removed in front of the printer. However, a jam of sheets should be removed in rear of the printer, and accordingly, its manipulatability is unsatisfactory.
In the printer disclosed in JP-A-2001-356548, a sheet tray is set in the upper part of the printer, and further, sheets are discharged onto a discharge tray in the upper part of the printer. In this configuration, a process cartridge in which a photosensitive drum and a developing means are integrally incorporated can be removed in front of the printer, and a jam of sheets can be removed at the top surface of the printer. However, since the sheet tray overhangs at the top surface of the printer, the occupied floor area is increased. Further, the capacity of supply of sheets cannot be increased in comparison with a printer in which a sheet tray is set in the lower part of the printer. It is noted that this document also discloses an example in which a sheet tray is set in the lower part of a printer. Even in this configuration, the process cartridge can be removed in front of the printer. However, a jam of sheets should be removed in rear of the printer, and accordingly, its manipulatability is low.
Thus, in the tandem system color printer, such a configuration that the image forming means are arranged in a vertical direction is used in order to miniaturize the printer, and in the case of selection of an intermediate image transfer medium system in order to accept various kinds of sheets, the manipulatability for replacement of toners and removal of a jam of sheet becomes lower.